So your saying, for example, on a hex buffer feed the sck into one buffer, then the output of that buffer to the input of the remaining 5. Use those 5 outputs to drive the 20 shift registers in groups of 4?

This would mean 2 hex buffers are required, 1 for sck and 1 for latch. Am I understanding that correctly?

Yes.Or just use one 74xx04 and feed one Arduino output to one buffer and the output of that to two others. That uses half the chip. Repeat for the other signal, then you have enough to drive two lots of 20 shift registers.